Connected: An Internet Encyclopedia
RFC 1812 - 5.3.3 IP Precedence

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### 5.3.3 IP Precedence

This section specifies requirements and guidelines for appropriate processing of the IP Precedence field in routers. Precedence is a scheme for allocating resources in the network based on the relative importance of different traffic flows. The IP specification defines specific values to be used in this field for various types of traffic.

The basic mechanisms for precedence processing in a router are preferential resource allocation, including both precedence- ordered queue service and precedence-based congestion control, and selection of Link Layer priority features. The router also selects the IP precedence for routing, management and control traffic it originates. For a more extensive discussion of IP Precedence and its implementation see [FORWARD:6].

Precedence-ordered queue service, as discussed in this section, includes but is not limited to the queue for the forwarding process and queues for outgoing links. It is intended that a router supporting precedence should also use the precedence indication at whatever points in its processing are concerned with allocation of finite resources, such as packet buffers or Link Layer connections. The set of such points is implementation- dependent.

DISCUSSION

  Although the Precedence field was originally provided for use in DOD systems where large traffic surges or major damage to the network are viewed as inherent threats, it has useful applications for many non-military IP networks. Although the traffic handling capacity of networks has grown greatly in recent years, the traffic generating ability of the users has also grown, and network overload conditions still occur at times. Since IP-based routing and management protocols have become more critical to the successful operation of the Internet, overloads present two additional risks to the network:

  1. High delays may result in routing protocol packets being lost. This may cause the routing protocol to falsely deduce a topology change and propagate this false information to other routers. Not only can this cause routes to oscillate, but an extra processing burden may be placed on other routers.

  2. High delays may interfere with the use of network management tools to analyze and perhaps correct or relieve the problem in the network that caused the overload condition to occur.

  Implementation and appropriate use of the Precedence mechanism alleviates both of these problems.

5.3.3.1 Precedence-Ordered Queue Service
5.3.3.2 Lower Layer Precedence Mappings
5.3.3.3 Precedence Handling For All Routers

Next: 5.3.3.1 Precedence-Ordered Queue Service

Connected: An Internet Encyclopedia
RFC 1812 - 5.3.3 IP Precedence